Directed energy weapons and specifically high-energy laser (HEL) weapons are being considered for variety of military applications with respect to a variety of platforms, e.g., spaceborne, airborne and land based systems to name a few. These weapons generally involve the use of the laser or other source of a high-power beam to track and destroy a target. To achieve mission objectives, directed energy weapons must be accurately steered and optimally focused. Steering involves line-of-sight control and focusing, with respect to HEL weapons, involves wavefront error correction. Currently, wavefront error correction is typically achieved using adaptive optics. For example, use of adaptive optics in laser beam control requires placing one or more deformable mirrors within the highest intensity portion of the beam path. The conventional deformable mirror is typically a large element with a thin face sheet and a number of piezoelectric actuators. Actuators are located behind the face sheet and are electrically driven to push and pull on the surface thereof to effect the deformation required to correct wavefront errors in an outgoing beam. The size of the active region of the deformable mirror must accommodate the full size of the high-power laser beam in the high-power Coudé path prior to expansion via an output telescope.
In addition, one or more fast-steering mirrors may be used to correct for tilt and direct the line-of-sight. A coarse gimbal may be employed to correct for line-of-sight errors as well. A plurality of wavefront sensors are typically employed along with an Aperture Sharing Element (ASE). The ASE allows a single shared aperture to be advantageously used for both the low-power sensors and the high-power output laser beam, ensuring that the path through the atmosphere taken by the high-power beam is the same as that taken by the wavefront sensor and that the correction applied to the shared atmospheric path is optimal for the high-power beam. In at least some high-power applications, the ASE includes both reflective and refractive properties. Placement of the ASE in the path of the high-power beam subjects it to errors arising from distortion of the optical properties of the ASE caused by non-uniform heating. In some applications, ASE errors observed by the low-power sensors differ from those encountered by the high-power beam, limiting a system's ability to correct for such so-called non-common path errors.
In U.S. Pat. No. 5,694,408, issued Dec. 2, 1997, the entire teachings of which are incorporated herein by reference, Bott, Rice, and Zediker appear to disclose a scheme which allows the deformable element to be placed in the low intensity region between a master oscillator and an array of fiber power amplifiers. The approach is to pre-distort the phase of the oscillator beamlets after separation in a distribution network and before injection into the fiber amplifier array, such that the pre-distortion corrects both the position error between the individual fibers and optical aberrations in the atmosphere. However, this scheme is practical only with a coherently-combined array of single-mode fiber amplifiers, as each fiber channel is correctable in piston only, not high order. Also, this scheme is not applicable to multi-mode laser media such as large core fiber amplifiers or bulk media lasers as contemplated for weapon class HEL devices and may not be scaleable to high-power levels due to random, high frequency phase noise caused by pump-induced temperature fluctuations within the fibers.
In U.S. Pat. No. 5,090,795, issued Feb. 25, 1992, the entire teachings of which are incorporated herein by reference, O'Meara and Valley appear to disclose several related schemes for using a liquid crystal light valve (LCLV) in a self-correcting adaptive optics system. This approach, however, places the LCLV in the high-power beam path and is therefore limited by the damage susceptibility of the liquid crystal material.
The need was addressed by U.S. Pat. No. 6,809,307 issued Oct. 26, 2004 to Byren et al. and entitled “System and Method for Effecting High-Power Beam Control With Adaptive Optics in Low Power Beam Path,” hereinafter the ‘Byren’ patent, the entire teachings of which are incorporated herein by reference. In the Byren patent, a beam control system and method that utilizes the wavefront reversal property of nonlinear optical phase conjugation to permit incorporation of a liquid crystal Optical Phased Array (OPA) within low-power legs of the beam control system is disclosed and claimed. The heart of the cited invention is the use of deformable optical elements in the low-power path of a High-Energy Laser (HEL) beam control subsystem to correct for atmospheric turbulence, aero-optic effects, and HEL beam path aberrations.
The Byren patent is adapted for use with HEL devices incorporating phase conjugating mirrors. Unfortunately, requiring that HEL sources include phase conjugating mirror impose limitations on the types of HEL devices (i.e., an inability to use those without phase conjugate mirrors).
Hence, a need remains in the art for a system or method for a fast, large-stroke, high spatial bandwidth or high order system or method for effecting wavefront correction of a high-power beam without limitation as to selection of the high-energy source.